Start-up procedure for a brushless, sensorless motor

ABSTRACT

A method for starting-up in a desired forward sense of rotation a multiphase, brushless, sensorless, DC motor, while limiting the extent of a possible backward rotation. First, a predetermined initial phase is excited (thereby accelerating the rotor toward an equilibrium position for that initial phase), for only a fraction of the time necessary for the accelerated rotor to travel through a nearest angular position which would determine a &#34;zero-crossing&#34; in the waveform of any one of the back electromotive forces (BEMFs) which are induced by the rotor on the windings of the motor. After the elapsing of this brief impulse of excitation, the sign of the BEMFs induced in the windings of the motor are digitally read thus producing a first reading. The occurrence of a first &#34;zero-crossing&#34; event is monitored, and, if this happens within a preset interval of time subsequent to the instant of interruption of the first excitation impulse, the optimal phase to be excited first for accelerating the motor in the desired direction is decoded through a look-up table, and the start-up process may proceed. If such a zero-crossing occurrence is not detected within said period of time, the routine is repeated by exciting a different phase, which is functionally shifted by two phase positions from the initial phase. The maximum backward rotation that may occur in the worst of cases is sensibly less than the angular distance which separates two adjacent equilibrium positions of the rotor and in practice may be of just few degrees.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a technique for starting anelectronically switched, brushless, multi-phase, DC motor, without rotorposition sensors.

The use of brushless DC motors is increasingly popular because of thelow electrical noise generated by these motors. In such motors, apermanent-magnet rotor is typically used, and switching transistorsdrive current through the various stator windings. Commonly three statorwindings are used, connected in a star-configuration. By connecting oneof the three stator windings to a current source and another to acurrent sink, six different phases of stator magnetization can bedefined.

If position-sensing devices (such as Hall-effect or optical sensors) areused to detect the instantaneous shaft position, then electronicswitching can completely substitute for the commutation functionsformerly performed by brushes. However, the sensors themselves add acost and reliability penalty. Thus, substantial work has been invested,during the past decade, in eliminating the use of position sensors insuch motors.

Once the motor is operating, the rotor position can be detected from theinduced voltage on the stator windings (which will vary depending on theposition and velocity of the rotor). Such back electromotive forces(BEMFs) can be detected differentially, with reference to the appliedvoltages. By processing these signals in order to determine the actualposition of the rotor, and accordingly synchronizing switching of theexcitation current through the phase-windings, the motor can beefficiently commutated.

Many systems are known for processing signals representative of theseinduced back electromotive forces, each having intrinsic advantages anddrawbacks.¹

However, all such sensorless motors face a start-up problem: because theinduced electromotive forces signals are not present when the motor isat rest, the starting position of the rotor is unknown. For this reason,several start-up procedures have been developed in order to overcomethis technical difficulty.

A first, known, start-up procedure consists in initially exciting acertain winding, in order to call the rotor toward an equilibrium point(null-torque position) relative to said excited phase.² After a numberof oscillations of the rotor about the equilibrium point, the rotoreventually stops in such a pre-established startup position. Thereafter,by selecting this initial excitation for start-up in a desired directionwith maximum torque, the motor may be started in an optimal manner.

The principal drawbacks of this start-up procedure are a possiblebackward rotation during the phase of alignment of the rotor with thecertain, fixed, start position, and a relatively long time required bythe start-up procedure.

According to another known start-up procedure, the phase-windings of themotor are excited sequentially several times at a variable frequency. Bystarting with a certain frequency and by increasing the frequency so asto force the rotor to follow in an open-circuit mode the excitationsequence of the phases, the rotor accelerates until it reaches a speedat which induced back electromotive force signals (BEMFs) may bedetected and processed.

The drawback of this procedure is that the rotor may not properly followthe excitation sequence, and may tend to oscillate about severalequilibrium positions or to rotate backward.

A third, known, start-up procedure consists in measuring the inductanceand the mutual inductance of the phase-windings of the motor. From themeasured values it is possible to determine the actual position of therotor.

A drawback of this procedure is that it is based on the results ofmeasurements which depend from the particular construction of the motorand therefore the system, which is relatively complex, must be adapted,case by case, to the type of motor.

Other known start-up procedures are based mainly on a sequentialexcitation under open-circuit conditions, and differ among each other inthe manner the accelerating sequence and the repetition thereof arecarried out.

In many applications, backward rotation at startup must be avoided. Forexample, in magnetic disk drives for computers, a non-negligiblebackward rotation may damage the reading heads.

Thus, there is still a need for a fast, start-up procedure which willprevent backward rotation, and which is practically independent of thefabrication characteristics of the motor, and which can be implementedin an integrated circuit, preferably in the form of hard-wired logic.

These objectives are achieved with the start-up system provided by thepresent invention, which implements a start-up procedure whichcomprises: exciting a predefined phase to call the rotor toward anequilibrium point relative to said excited phase, for a pre-establishedfraction of the time necessary to the accelerated rotor, depending onits inertia characteristics, to reach a nearest position at which theBEMF which is induced by the motion of the rotor in any one of thephase-windings of the motor undergoes a "zero-crossing" (change ofsign), and, before the occurrence of such a nearest "zero-crossing"event, digitally reading the output configuration of a plurality ofcomparators which assess the induced BEMFs on the phase-windings of themotor, detecting thereafter the occurrence of such a first"zero-crossing" event by one of said induced BEMFs within apre-established period of time following the instant of saidinterruption of said first excitation; if said "zero-crossing" eventoccurs, reading a second time the output configuration of saidcomparators and decoding, by processing said first and second readings,the position of the rotor in respect to the phase-windings of the motor,and therefore the phase to be excited next in order to accelerate therotor in the desired sense of rotation with a maximum torque; if said"zero-crossing" event does not occur before the termination of saidpre-established period of time following the instant of interruption ofsaid first excitation current pulse, repeating the process by exciting adifferent phase of the motor which is functionally shifted angularly bytwo phases as compared to said predefined phase.

In practice, in the worst case which may exist at the start-up instant,the maximum backward rotation of the rotor which may occur is less thanan angle of rotation equivalent to the angular separation between twopoles (or equilibrium positions) of the motor. For example, in the caseof a motor having 36 equilibrium positions, the maximum possiblebackward rotation which may occur in the worst of cases, will be limitedto less than (360/36) 10 degrees. In practice a maximum back rotation ofabout 7-8 degrees is experienced. According to a preferred embodiment,the start-up procedure further comprises a preliminary step during whichall the phases of the motor are sequentially excited one or severaltimes, each phase for a fraction of said pre-established duration ofsaid first excitation step of the predefined phase, in order toeliminate the static component of friction, without actually moving therotor from its rest position. This preliminary sequential excitation ofthe phases for periods of time insufficient to move the rotor from itsrest position, determines a mechanical preconditioning of the motorwhich will facilitate the ensuing performance of the real start-upprocedure.

The disclosed innovations provide methods, circuits and systems forstarting-up in a desired forward sense of rotation a multiphase,brushless, sensorless, DC motor, while limiting the extent of a possiblebackward rotation. First, a predetermined initial phase is excited(thereby accelerating the rotor toward an equilibrium position for thatinitial phase), for only a fraction of the time necessary for theaccelerated rotor to travel through a nearest angular position whichwould determine a "zero-crossing" in the waveform of any one of the backelectromotive forces (BEMFs) which are induced by the rotor on thewindings of the motor. After the elapsing of this brief impulse ofexcitation, the sign of the BEMFs induced in the windings of the motorare digitally read thus producing a first reading. The occurrence of afirst "zero-crossing" event is monitored, and, if this happens within apreset interval of time subsequent to the instant of interruption of thefirst excitation impulse, the optimal phase to be excited first foraccelerating the motor in the desired direction is decoded through alook-up table, and the start-up process may proceed. If such azero-crossing occurrence is not detected within said period of time, theroutine is repeated by exciting a different phase, which is functionallyshifted by two phase positions from the initial phase. The maximumbackward rotation that may occur in the worst of cases is sensibly lessthan the angular distance which separates two adjacent equilibriumpositions of the rotor and in practice may be of just few degrees.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIGS. 1, 2 and 3 illustrate the operation of a star-configured threephase motor, having N number of poles for six distinct phases ofexcitation;

FIG. 4 is a partial block diagram of the three differential amplifiersand of the three comparators connected in cascade to the differentialamplifiers, which are used for amplifying and evaluating the signs ofthe BEMFs induced in the three windings of the motor.

FIGS. 5 and 6 show the manner in which the output configuration of thethree comparators of the block diagram of FIG. 4 is digitally read inorder to produce bit-to-bit complementary data for the two senses ofrotation.

FIG. 7 depicts a "look-up" decode table for the optimal phase to beexcited in order to accelerate the motor in the desired direction or tobrake the rotor, should it have been initially started in an opposite(backward) direction, in function of the data read from the comparatorsbefore and after a "zero-crossing" event;

FIG. 8 is a flow diagram of the start-up routine of the presentinvention;

FIGS. 9, 10, 11, 12, 13, 14 and 15 show how the start-up routine of theinvention is implemented in the different possible situations that mayoccur.

FIG. 16 is a block diagram of an embodiment of a start-up system of theinvention.

FIG. 17 is a more detailed block diagram of the start-up system FIG. 16.

FIG. 18 shows greater detail of the connection of output phase buffer 7to the switching transistors which actually source or sink current tothe motor, in a sample preferred embodiment.

FIG. 19 shows a sample embodiment of the innovative motor control systemon an integrated circuit.

FIG. 20 shows a sample embodiment of a innovative motor control system,including a microcontroller in combination with an array of driverdevices. (In a sample embodiment, the driver devices are provided by anL6232 part from SGS-Thomson.)

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferredembodiment. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily delimit anyof the various claimed inventions. Moreover, some statements may applyto some inventive features but not to others.

The set of figures refer to the case of a three-winding motor, in a starconfiguration, i.e. having six different, excitable phases, and an Nnumber of poles. Therefore, there will be 6×N equilibrium points in acomplete revolution of the rotor. In the following description, eachexcitation phase is indicated with two capital letters: a first letter(A, B or C) designates the winding through which the currentconventionally flows from a supply terminal toward the star center (CT),a second capital letter, preceded by a "!" sign (or topped by ahorizontal bar), designates the winding through which the current,coming from the star center (CT) flows toward another supply terminal(re: FIG. 1).

For each excited phase, the rotor tends to dispose itself in coincidencewith one of the stable equilibrium points relative to the particularphase of excitation, therefore for a total of six phases there will be6×N equilibrium points in a complete revolution and there will be asmany points of mechanically unstable equilibrium which will be shiftedby 180 electrical degrees from the respective stable equilibrium point.

For example, by exciting the phase A!B, the equilibrium and instabilitypoints of the rotor will be those indicated in FIG. 2, which shows thetorque curves of the motor in relation to the different phases ofexcitation. When the motor stops and the electrical excitation ceases,the rotor may casually align itself with any one of the differentequilibrium points. For each of these rest positions of the rotor therewill be a phase which, if excited, will produce upon the rotor a maximumstart-up torque in a certain direction of rotation. For example, if therotor is at rest in a stable equilibrium position relative to the phaseA!B (as shown in FIG. 2), the excitation of the six different phase ofthe motor would produce a torque on the rotor according to the followingscheme:

    ______________________________________                                        Excited Phase Instantaneous Relative Impulse of Torque                        Start-up Torque as Depicted in FIG. 3                                         ______________________________________                                        A!C                -√3/2 Tp                                            B!C                -√3/2 Tp                                            B!A                0 Tp                                                       C!A                +√3/2 Tp                                            C!B                +√3/2 Tp                                            A!B                0                                                          ______________________________________                                    

Tp represents the peak value of the torque curves and such a value iscorrelated, through a torque constant of the motor (Kt, in units ofNm/A), to the excitation current of the motor. The (-) signconventionally means that the torque will start-up the rotor in abackward direction of rotation.

From the above table, and by observing also the respective torque curvesof FIG. 3, as well as the corresponding curves of the three inducedBEMFs which are also shown therein, it is evident that the phase to beexcited first in order to achieve the maximum start-up torque in aforward direction of rotation is the phase C!A.

Moreover it is evident that by exciting casually one of the six phaseswhile the rotor is at rest and casually aligned with any of thedifferent equilibrium points, either a forward motion or a backwardmotion or no motion at all may occur.

As shown in the partial block diagram of FIG. 4, the BEMFs induced bythe rotation of the rotor on the windings of the motor, namely: BEMF-A,BEMF-B and BEMF-C, may be detected and amplified by the use of the(conventionally connected) differential amplifiers A, B and C. Theoutput of these differential amplifiers is evaluated by the use ofrespective hysteresis comparators: COMP-A, COMP-B and COMP-C, which maybe for example "Schmitt-trigger" comparators. The readable digital datagiven by the output configuration of the comparators of FIG. 4 may beutilized for establishing the actual position of the rotor and itsdirection of rotation if the motor is disexcited for a certain intervalof time during the reading.

In fact, as may be observed in FIG. 5, the state (polarity) of the BEMFswill not change between two successive "zero-crossing" events, thereforeany position of the rotor between two subsequent "zero-crossing" eventsis unequivocally represented by a certain state of the output of thecomparators. Therefore, if the output configuration of the comparatorsis digitally read and if a "0-weight" bit is associated with thecomparator relative to the phase-winding A, a 1-weight bit to thecomparator relative to the phase-winding B and the 2-weight bit to thecomparator relative to the phase-winding C, a data having a valuecomprised between 0 and 7 may be derived by reading the outputconfiguration of the three comparators, as depicted in FIG. 6.

In the table at the base of the diagrams representing the BEMFs of FIG.5, the read values of the output configuration of the three comparatorsof FIG. 4 are indicated for the different possible positions of therotor and for a given sense of rotation.

As it may be observed, a certain data corresponding to a relative outputconfiguration of the comparators is assigned to each position of therotor between two subsequent "zero-crossing" positions and this data isdifferent depending upon the sense of rotation of the rotor in reachingsaid position.

The data corresponding to the output state of the comparators for eachposition of the rotor moving in a forward direction is in practicebit-to-bit complementary to the data in case backward rotation. Thisbecause of the fact that the sign of the BEMF signals is inverted whenthe motor changes direction of rotation. For example, if at a certaininstant the rotor is aligned with the phase A!B, the data read will be 2if the rotor is moving in a forward rotation or 5 if the rotor isrotating backward. This is due to the sign of the back electromotiveforces which are induced in the windings in one case and in the othercase.

This fact is important because when the motor changes direction ofrotation the data read at the output of the comparators are bit-to-bitcomplementary. This means that upon an inversion of rotation, a"pseudo-zero-crossing" event is generated and this"pseudo-zero-crossing" event is fundamental for the start-up algorithmof the method of the present invention, as will be more fully describedhereinafter. In function of the addresses (codes) obtained in the formof digital data corresponding to the output configuration of thecomparators, before and after a "zero-crossing" event, and whichunequivocally identify the actual position and sense of rotation of therotor, it is possible to employ a so-called "look-up table" (orcode-table) purposely created for decoding (identifying) the optimalphase of excitation in order to start-up the motor in the desireddirection (forward rotation) with a maximum start-up torque. For theexample shown, such a "look-up table" is schematically illustrated inFIG. 7. The start-up process of the invention, according to a preferredembodiment, may be illustrated in the form of an algorithm by theflow-chart of FIG. 8.

Preferably, before initiating the start-up procedure, a brief sequentialexcitation of all the phases of the motor may be carried out once orseveral times, by sequentially exciting each phase for an extremelyshort time, insufficient to start-up the rotor, in order to eliminate orreduce static friction and therefore place the motor, still at rest, ina more favorable condition for performing a real start-up routine. Ofcourse, this "preconditioning" phase is not strictly necessary and mayalso not be carried out. The real start-up procedure begins by excitinga predefined phase of the motor (it is irrelevant which phase ispredefined and in the following description and in the set of exampleswhich follows the phase C!A is supposed to be the predefined phase). Thepredefined phase is excited for a preset period of time T0, the lengthof which represents the only parameter which must be adapted to theinertial characteristics of the system which is associated with therotor of the motor. The larger the inertia of the system, the longershould be this preset time T0. The time T0 must be sufficiently long forthe impulse of torque which is imparted to the rotor to be able to move(accelerate) the rotor from its rest position toward an equilibriumposition relative to said predefined first excited phase. On the otherhand the time T0 must be short enough to elapse before the rotor,eventually accelerated toward a relative equilibrium position, hastravelled by an angular distance long enough to cause the occurrence ofa first "zero-crossing" event in the waveform of any one of the backelectromotive forces which are induced in the windings of the motor,i.e. by an angular distance equivalent to 1/2 the angular separationdistance between two pulse or equilibrium positions. At the end of thepreset period of time T0, said first excitation of the predefined phaseis interrupted, leaving the rotor to proceed by inertia toward theequilibrium position relative to the excited phase and at that point,after a masking time sufficiently long for the transient phenomenafollowing the interruption of the excitation current to decay, a firstreading of the output configuration of the comparators is effected(eventually recording the data read) before the rotor, so accelerated bythe brief impulse of torque received, reaches a position such as toproduce a first "zero-crossing" event in any one of the induced BEMFs.

After the eventual occurrence of a first "zero-crossing" event in aninduced BEMF, the changed output configuration of the comparators isread again. The data relative to the two successive readings of theoutput configuration of the comparators, before and after the occurrenceof a first "zero-crossing" event in the waveform of the induced BEMFs,respectively, are utilized as decoding addresses (codes) of a "look-uptable". The first reading decodes the row and the second reading decodesthe column of the "look-up table", or vice versa. The phase as decodedthrough the "look-up table", represents the optimal phase to be excitedin order to start-up the motor in the desired direction (forwardrotation) with a maximum start-up torque. By observing the "look-uptable" of FIG. 7, it is easily recognized that the optimal phase ofexcitation is practically determined by the first reading alone whichdecodes the row. In fact the decoding values belonging to the same roware all the same. The second reading, which is effected after a first"zero-crossing" event, is necessary as a confirmation of the firstreading. In fact, if the value read during a first reading is forexample 0-2 (which decodes the third row of the table) then the secondreading should give 0-6 or 0-3 (i.e. must decode the fourth or the sixthcolumn of the table). If this does not happen, it means that someinconvenience may have occurred (for example, a false first reading dueto a persistence of transient phenomena). As a consequence a code (off)is decoded which will signal to the system the occurrence of a falsereading so that the system may suitably react, e.g. by repeating thestart-up routine.

After the second reading confirms correctness the start-up routine ispractically terminated and the motor may be accelerated and controlledby any suitable technique.

Of course, the initial rest position of the rotor as referred to thepredefined phase of first excitation may be such that the rotor will notundergo any acceleration during the pre-established period of time T0 ofthe first excitation. In such a case, notwithstanding the first readingof the output configuration of the comparators after elapsing the timeT0, no "zero-crossing" event will be detected within a certainpre-established delay. Such a missed detection of a "zero-crossing"event will automatically determine the repetition of the start-uproutine by exciting a phase different from the predefined phase andwhich is functionally shifted by two phase-intervals from the predefinedphase.

Preferably, the instant of interruption of the first excitation of thepredefined phase for a preset time T0 is masked for a period of time ofa fraction of T0, e.g. 1/8 T0, in order to wait for a substantial decayof transients, before effecting the reading of the back electromotiveforces.

The procedure is illustrated in more detail in the flow chart of FIG. 8.

It is evident that in case the initial rest position of the rotor inrespect to the predefined phase of first excitation is such as todetermine the movement of the rotor toward a relative equilibriumposition in a backward sense of rotation, the decoding of the optimalphase of excitation for starting-up in a forward direction the motor andthe excitation of a so identified phase will produce an immediatebraking action of the inertial backward movement of the rotor and anacceleration in the desired forward sense of rotation. In practice thisprevents a possible backward rotation of the rotor from progressingbeyond an angular distance equivalent to less than the angular distanceof separation between two successive equilibrium positions of the rotor.In practice, this means ensuring a possible maximum backward rotation ofjust few degrees (e.g. <10°).

DESCRIPTION OF THE BEST MODE

The different possible situation at start-up and the relative behaviorof the motor in performing the start-up routine object of the presentinvention will now be examined in detail, by referring to the relativefigures.

Start-up of the Rotor from an Equilibrium Position Relative to Phase C!B

With reference to FIG. 9, the rotor is supposed to be in an initial,rest position corresponding to an equilibrium point relative to thephase C!B. By exciting the C!A phase for a period of time T0, the rotoris subjected to an impulse of torque given by the shaded area and therotor is accelerated in a forward direction of rotation. Once the timeT0 has elapsed, the excitation is interrupted and, after a sufficientlylong masking time (e.g. Tmask=1/8*T0), the output configuration of thecomparators is read and the data which is obtained has value 6, as shownin the table at the bottom of the diagrams.

After this first reading, with an inertial progression of the rotorrotation, a first "zero-crossing" of the signal representative of theBEMF across the C winding will be detected. After the occurrence of thisfirst "zero-crossing" event, the output configuration of the comparatorsis read again and gives a value 2. Therefore the complete address is6-2, wherein 6 decodes the row and 2 decodes the column of the "look-uptable". The decoded address makes a phase pointer to point the phase B!Cas the phase to be excited first in order to accelerate the motor in thedesired sense of rotation (forward rotation).

In the case described above, the motor is started in a forward directionof rotation without undergoing any backward rotation.

Start-up of the Rotor from an Equilibrium Position Relative to Phase A!B

With reference to FIG. 10, the rotor is supposed to be in an initial,rest position corresponding to an equilibrium point relative to thephase A!B. By exciting the C!A phase for a period of time T0, the rotoris subjected to an impulse of torque given by the shaded area and therotor is accelerated in a forward direction of rotation. Once the timeT0 has elapsed, the excitation is interrupted and, after a sufficientlylong masking time (e.g. Tmask=1/8*T0), the output configuration of thecomparators is read and the data which is obtained has value 4, as shownin the table at the bottom of the diagrams.

After this first reading, with an inertial progression of the rotorrotation, a first "zero-crossing" of the signal representative of theBEMF across the B winding will be detected. After the occurrence of thisevent, the output configuration of the comparators is read again andgives a value 6. Therefore the complete address is 4-6, wherein 4decodes the row and 6 decodes the column of the "look-up table". Thedecoded address makes a phase-pointer to point the phase B!A as thephase to be excited first in order to accelerate the motor in thedesired sense of rotation (forward). Also in the case described above,the motor is started in a forward direction of rotation withoutundergoing any backward rotation.

Start-up of the Rotor from an Equilibrium Position Relative to Phase C!A

In view of the fact the rotor is at rest in an equilibrium pointrelative to the predefined phase of first excitation itself, it wouldnot receive any acceleration because the applied impulse of torque willbe null. This particular situation will be eventually recognized by thesystem by a decrement to zero of a counter loaded with a value equal toa multiple number of T0 intervals of time, e.g. 8*T0.

If no "zero-crossing" event in any one of the (BEMF) induced in themotor windings is detected before the decrement to zero of theabove-mentioned counter, a different phase of the motor is excited forthe same preset period of time T0. If the predefined phase of firstexcitation is C!A, the new phase to be excited in order to repeat theroutine will be a phase shifted by two phase-intervals from the presetphase, i.e. the B!C phase.

With reference to FIG. 11, the rotor is supposed to be in an initial,rest position corresponding to an equilibrium point relative to thephase C!A. By exciting the B!C phase for a period of time T0, the rotoris subjected to an impulse of torque given by the shaded area and therotor is accelerated in a forward direction of rotation. Once the timeT0 has elapsed, the excitation is interrupted and, after a sufficientlylong masking time (e.g. Tmask=1/8*T0), the output configuration of thecomparators is read and the data which is obtained has value 2, as shownin the table at the bottom of the diagrams.

After this first reading, with an inertial progression of the rotorrotation, a first "zero-crossing" of the signal representative of theBEMF across the A winding will be detected. After the occurrence of thisevent, the output configuration of the comparators is read again andgives a value 3. Therefore the complete address is 2-3, wherein 2decodes the row and 3 decodes the column of the "look-up table". Thedecoded address makes a phase-pointer to point the phase A!C as thephase to be excited first in order to accelerate the motor in thedesired sense of rotation (forward). Also in the case described above,the motor is started in a forward direction of rotation withoutundergoing any backward rotation.

What follows now is the description of three cases where a firstexcitation step of the predefined phase for a time T0 produces abackward acceleration of the rotor.

Start-up of the Rotor from an Equilibrium Position Relative to Phase B!C

With reference to FIG. 12, the rotor is initially at rest in anequilibrium position relative to the B!C phase. By exciting thepredefined phase C!A for a time T0, the rotor receives an impulse oftorque given by the shaded area indicated with 1 of the diagram whichcauses an acceleration of the rotor in a backward sense of rotation.

At the end of the T0 period, the excitation is interrupted and, afterthe usual masking time Tmask (e.g. Tmask=1/8*T0) the outputconfiguration of the comparators is read and gives the value 6. Afterthe system has detected a first "zero-crossing" event in the BEMFinduced on the B winding of the motor, the new output configuration ofthe comparators is read and gives the value 4.

Therefore the complete address is 6-4, wherein 6 decodes the row and 4decodes the column of the "look-up table". The decoded address makes thephase-pointer to point the B!C phase, as the phase to be excited first.By observing FIG. 12, it may be easily seen that by having recognized a6 to 4 transition in the output configuration of the comparators and, asa consequence, having excited the relative B!C phase, the impulse oftorque which is imparted on the rotor and which is put in evidence bythe shaded area identified with 2 in the diagram, has the effect ofimmediately stopping the back rotation of the rotor when a first portionof the shaded area 2 under the torque curve equals the area 1 and ofinverting the sense of rotation of the rotor to a forward direction, inview of the fact that the area 2 is always greater than the area 1.Therefore the motor stops and changes direction.

Upon the change of direction of rotation of the rotor, the relative backelectromotive forces (BEMFs) also switch polarity. This represents a"pseudo-zero-crossing" event, which in practice is interpreted as suchby the system and, as a consequence, the system determines theexcitation of a next optimal phase which is the A!C phase. In thismanner, the motor accelerates in the desired forward direction ofrotation under the new impulse of torque which is represented by theshaded area, identified with 3 in the diagram of FIG. 12.

The routine may be described as follows:

a) exciting the C!A phase, thus producing an impulse of torque given bythe area 1;

b) detecting the occurrence of a first "zero-crossing" event;

c) decoding, by using the data read off the comparators before and afterthe detected "zero-crossing" event, the phase which will exert themaximum braking torque and exciting said phase B!C;

d) when the area 2 of the impulse of braking torque equals the area 1,the rotor will come to a stop and change direction of rotation thuscausing an inversion of sign of the induced BEMFs which in turn isinterpreted by the system as a new "zero-crossing" event;

e) accelerating the motor thus started in a forward rotation by excitinga newly decoded phase.

Start-up of the Rotor from an Equilibrium Position Relative Phase B!A

With reference to FIG. 13, the rotor is initially at rest in anequilibrium position relative to the B!A phase. By exciting thepredefined phase C!A for a time T0, the rotor receives an impulse oftorque given by the shaded area indicated with 1 of the diagram whichcauses an acceleration of the rotor in a backward sense of rotation.

At the end of the T0 period, the excitation is interrupted and, afterthe usual masking time Tmask, (e.g. Tmask=1/8*T0) the outputconfiguration of the comparators is read and gives the value 4. Afterthe system has detected a first "zero-crossing" event in the BEMFinduced on the A winding of the motor, the new output configuration ofthe comparators is read and gives the value 5.

Therefore the complete address is 4-5, wherein 4 decodes the row and 5decodes the column of the "look-up". The decoded address makes thephase-pointer to point the B!A phase, as the phase to be excited first.By observing FIG. 13, it may be easily seen that by having recognized a4 to 5 transition in the output configuration of the comparators and, asa consequence, having excited the relative B!A phase, the impulse oftorque which is imparted on the rotor and which is put in evidence bythe shaded area identified with 2 in the diagram, has the effect ofimmediately stopping the back rotation of the rotor when a first portionof the shaded area 2 under the torque curve equals the area 1 and ofinverting the sense of rotation of the rotor to a forward direction, inview of the fact that the area 2 is always greater than the area 1.Therefore the motor stops and changes direction.

Upon the change of direction of rotation of the rotor, the relative backelectromotive forces (BEMF) also switch polarity. This represents a"pseudo-zero-crossing" event, which in practice is read as such by thesystem and, as a consequence, the system determines the excitation of anext optimal phase which is the B!C phase. In this manner, the motoraccelerates in the desired forward direction of rotation under the newimpulse of torque which is represented by the shaded area, identifiedwith 3 in the diagram of FIG. 13. The routine may be described asfollows:

a) exciting the C!A phase, thus producing an impulse of torque given bythe area 1;

b) detecting the occurrence of a first "zero-crossing" event;

c) decoding by using the data read off the comparators before and afterthe detected "zero-crossing" event the phase which will exert themaximum braking torque and exciting said phase B!A;

d) when the area 2 of the impulse of braking torque equals the area 1,the rotor will come to a stop and change direction of rotation thuscausing an inversion of sign of the induced BEMFs which in turn isinterpreted by the system as a new "zero-crossing" event;

e) accelerating the motor thus started in a forward rotation by excitinga newly decoded phase.

Start-up of the Rotor from an Equilibrium Position Relative to Phase A!C

In view of the fact the rotor is at rest in an equilibrium pointrelative to the predefined phase of first excitation, it would notreceive any acceleration because the applied impulse of torque will benull. This particular situation will be eventually recognized by thesystem by a decrement to zero of a counter loaded with a value equal toa multiple number of T0 intervals of time, e.g. 8*T0.

If no "zero-crossing" event in any one of the (BEMF) induced in themotor windings is detected before the decrement to zero of theabove-mentioned counter, a different phase of the motor is excited forthe same preset period of time T0. If the predefined phase of firstexcitation is C!A, the new phase to be excited in order to repeat theroutine will be a phase shifted by two phase-intervals from the presetphase, i.e. the B!C phase.

With reference to FIG. 14, the rotor is supposed to be in an initial,rest position corresponding to an equilibrium point relative to thephase A!C. By exciting the B!C phase for a period of time T0, the rotoris subjected to an impulse of torque given by the shaded area and therotor is accelerated in a forward direction of rotation. Once the timeT0 has elapsed, the excitation is interrupted and, after a sufficientlylong masking time (e.g. Tmask=1/8*T0), the output configuration of thecomparators is read and the data which is obtained has value 2, as shownin the table at the bottom of the diagrams.

After this first reading, with an inertial progression of the rotorrotation, a first "zero-crossing" of the signal representative of theBEMF across the C winding will be detected. After the occurrence of thisevent, the output configuration of the comparators is read again andgives a value 6. Therefore the complete address is 2-6, wherein 2decodes the row and 6 decodes the column of the "look-up table". Thedecoded address makes a phase-pointer to point the phase A!C as thephase to be excited first.

By observing FIG. 14, it may be easily seen that by having recognized a2 to 6 transition in the output configuration of the comparators and, asa consequence, having excited the relative A!C phase, the impulse oftorque which is imparted on the rotor and which is put in evidence bythe shaded area identified with 2 in the diagram, has the effect ofimmediately stopping the back rotation of the rotor when a first portionof the shaded area 2 under the torque curve equals the area 1 and ofinverting the sense of rotation of the rotor to a forward direction, inview of the fact that the area 2 is always greater than the area 1.Therefore the motor stops and changes direction.

Upon the change of direction of rotation of the rotor, the relative backelectromotive forces (BEMF) also switch polarity. This represents a"pseudo-zero-crossing" event, which in practice is read as such by thesystem and, as a consequence, the system determines the excitation of anext optimal phase which is the A!B phase. In this manner, the motoraccelerates in the desired forward direction of rotation under the newimpulse of torque which is represented by the shaded area, identifiedwith 3 in the diagram of FIG. 14. The routine may be described asfollows:

a) exciting the B!C phase, thus producing an impulse of torque given bythe area 1;

b) detecting the occurrence of a first "zero-crossing" event;

c) decoding by using the data read off the comparators before and afterthe detected "zero-crossing" event the phase which will exert themaximum braking torque and exciting said phase A!C;

d) when the area 2 of the impulse of braking torque equals the area 1,the rotor will come to a stop and change direction of rotation thuscausing an inversion of sign of the induced BEMFs which in turn isinterpreted by the system as a new "zero-crossing" event;

e) accelerating the motor thus started in a forward rotation by excitinga newly decoded phase.

For better understanding the validity of the process of the inventionfor the start-up situations of the last three examples, it may be usefulto further emphasize the important aspect represented by the fact thatby starting from any equilibrium position of the rotor, the maximumimpulse of torque which may be exerted in a forward direction ofrotation is consistently always greater of the maximum impulse of torquewhich may be theoretically exerted in a backward sense of rotation.Therefore an advantageous condition exists for immediately exerting alarge impulse of braking torque in those cases where the momentaryinitial excitation of the predefined phase of the start-up routineactually starts the motor in a backward rotation. This fact is put inevidence in FIG. 15, wherein it is clearly shown that the shaded areas 1and 3 are always smaller than the shaded areas 2 and 4, respectively.

As schematically illustrated in FIG. 16, the systems comprises a blockof comparators 2 capable of assuming an output configuration whichrepresents the signs of the back electromotive forces induced in therespective phase-windings of the motor 1. The digital data derived fromreading the output configuration of the comparator 2 are sent to adecoding circuitry 3 which, by the use of a "look-up table" is capableof providing an address code which identifies the optimum phase to beexcited in order to start-up the motor in the desired direction (forwardrotation). The circuitry of the block 4, to which the output signals ofthe comparators of the block 2 are sent, detects the occurrence of a"zero-crossing" event in the waveforms of the induced BEMFs andconsequently generates a control signal which is sent to the decodingblock 3, which will process the new digital datum representing theoutput configuration of the set of comparators (block 2) subsequent tothe detected "zero-crossing" event. The block 5, which may beinitialized by a start signal, contains the timers which control thecorrect sequence of steps to be performed in accordance with thealgorithm of the start-up process, which, upon the recognition of theoptimum phase to be excited by the decoding block 3 and upon aconfirmation of the correctness of execution of the routine provided bythe block 6, starts-up the motor through the block 7 which determinesthe correct sequence of excitation beginning from the decoded optimalstarting phase.

FIG. 17 is a more detailed block diagram of an embodiment of the systemof the invention, as depicted in the preceding FIG. 16. The principalblocks described in FIG. 16 are identified in FIG. 17 by the respectiveboundaries traced with a dash-line. When a start signal is received, theflip-flop FF1 is set and therefore the start-run signal is placed to alogic "1". This signal enables the phase-buffer block PB to receive thedata which decode the phase to be excited by the start-up buffer blockSB1 and, at the same time, starts the one-shot timer OST1, which issuitably loaded in order that its output remains at a high logic levelfor a time T0.

The rising front of the signal which is produced on the output of theOST1 block, enables the loading of the code of the first phase to beexcited in the phase-latch PL1 and enables the same through theflip-flop FF2.

The falling front of the pulse loads the code which stops the excitationof the motor in the phase-latch PL1.

When such a code is recognized by the phase-buffer (PB), the latterdelays by a fractionary interval of time (e.g. T0/8) the resetting ofthe flip-flop FF2 which then enables the phase-decoder PD, which effectsthe first reading.

At this point a "zero-crossing" event is waited for and the followingalternative conditions of operation may occur.

1) a "zero-crossing" event is detected before the running out of the8×T0 interval stored in timer T1. In this case the "zero-cross" detectorZCD enables the decoder PD to effect a second reading. After havingperformed a second reading, the decoder PD enables the phase-latch PL2and transfers the code of the optimal phase which is in turn transferredto the start-up phase-buffer SP1. The flip-flop FF1 is thereafter resetand therefore the start-run signal, by becoming low, disables thestart-up routine. 2) a "zero crossing" event is not detected before therunning out of the timer T1.

In this case, the "end-of-count" (EOC) of the timer (T1) sets to zerothe timer itself, resets OST1 and excites as a new, first excitationphase a phase which is functionally shifted angularly by two phases asreferred to the previous, first-excitation phase the routine isrepeated.

Sample Specific Implementation

For clarity, the accompanying Figures depict some details of a samplespecific implementation. However, of course, this specificimplementation, and its specific details, are not be any means necessaryfor practicing the claimed invention.

FIG. 18 shows greater detail of the connection of output phase buffer 7to the switching transistors which actually source or sink current tothe motor, in a sample preferred embodiment.

FIG. 19 shows a sample embodiment of the innovative motor control systemon an integrated circuit.

FIG. 20 shows a sample embodiment of a innovative motor control system,including a microcontroller in combination with an array of driverdevices. (In a sample embodiment, the driver devices are provided by anL6232 part from SGS-Thomson.) By driving the inputs INHA, INHB, INHC,INLA, INLB and INLC, the power MOS are switched on and off according tothe following table:

    ______________________________________                                                                                EXCITED                               INHA  INHB    INHC    INLA  INLB  INLC  PHASE                                 ______________________________________                                        0     1       1       0     1     0     AB                                    0     1       1       0     0     1     AC                                    1     0       1       0     0     1     BC                                    1     0       1       1     0     0     BA                                    1     1       0       1     0     0     CA                                    1     1       0       0     1     0     CB                                    ______________________________________                                    

With reference to the circuit diagram of FIG. 18, what themicrocontroller or the wired logic circuitry must do is to switch thelogic state in an intelligent manner (i.e. at the correct instant) ofthe signals that are fed to the inputs: INHA, B, C and INLA, B, C.

FIG. 18 shows greater detail of the connection of output phase buffer 7to the switching transistors which actually source or sink current tothe motor, in a sample preferred embodiment.

FIG. 19 is a block diagram which shows a sample implementation of theinvention in a L6238 chip. This chip implements a so-called"alignment-and-go" start-up procedure. A contemplated advantageousembodiment is to retain the existing structure of the chip andsubstitute the logic circuit embodying the start-up procedure (indicatedby the arrow in the chip's diagram) with the circuit of the presentinvention to implement the improved start-up procedure.

The first time interval of excitation must necessarily be long enough tomove a rotor from the rest position, but this first time interval shouldpreferably conclude before a first zero-cross event occurs. Indeed, thisis the only parameter that needs to be adjusted in dependence on theactual characteristics of the motor.

Depending on the definition of the this first time interval, it mayoccur that the first zero-crossing event cannot be read. In this casethe motor would nevertheless start-up upon the occurrence of a secondzero-crossing event; however the resulting back-rotation (when present),would be incremented by a quantity equal to the angle between twosuccessive zero-cross positions.

In a sample demonstrated embodiment, the described innovations have beentested with different motors. For example, with a NIDEL 5 V motor havinga Kt value of 8.2 mV/(rad/sec), a time T0 of 20 msec was used, with astarting current I_(start) of 0.5 A.

Other tested motors were contained in hard disk assemblies of variousmanufacturers: among them a 2.5" hard disk of Western Digital, havingthe following characteristics:

V_(alim) =5 V;

Kt=5.55 mV/(rad/sec);

T0=20 msec;

I_(start) 0.5 A.

Other motors were assembled in VCR drives (Philips, Thomson, Panasonic).Only the excitation time T0=20 m sec, the supply voltage V_(alim) =12 Vand the start current I_(start) =1A of these motors were known.

The Kt values of the motors that may be equipped with the controller ofthe invention are normally such as to produce BEMF signals of sufficientamplitude to permit detection of zero-cross events at start-up. Once themotor has been started-up and is running, the BEMFs are clamped by theinput stage of the op-amps. Once the start-up procedure has successfullyterminated, the control of the running motor is performed by a differentcircuit.

Further Modifications and Variations

It will be recognized by those skilled in the art that the innovativeconcepts disclosed in the present application can be applied in a widevariety of contexts. Moreover, the preferred implementation can bemodified in a tremendous variety of ways. Accordingly, it should beunderstood that the modifications and variations suggested below andabove are merely illustrative. These examples may help to show some ofthe scope of the inventive concepts, but these examples do not nearlyexhaust the full scope of variations in the disclosed novel concepts.

For example, the specific assignment of the separate comparator outputsto different bit positions is of no importance at all. These bits merelyserve to identify a unique place in the lookup table, and the sequencingof entries in this table is unimportant.

For example, the preferred embodiment has been described in the specificcontext of a motor which has only three stator windings, and which usesa permanent-magnet rotor with only two poles. However, the disclosedinnovations can also be adapted to motors which include more than threestator windings, or even to motors which include a quadrupole rotor.

For another example, it is not by any means necessary that the motorwindings should be center-tapped as shown in FIG. 1. Although this is acommon and useful arrangement, the invention could also be applied to aconfiguration where the two terminals of each winding were separatelybrought out. Such a configuration would presumably be less advantageous,but could still benefit from the disclosed innovations.

For another example, the disclosed innovations would be equallyapplicable to a multi-phase DC motor which used a higher number ofphysical windings per stator terminal than the presently preferredembodiment.

For another example, the disclosed innovations can also be implementedusing a microcontroller (or other programmable logic) instead of thehard-wired logic implementation of the presently preferred embodiment.

The selection of switching transistors is merely dictated by the voltageand current requirements of the motor used. Although the disclosedinnovations are particularly useful in small motors (where the extracost of position sensing is a significant factor), these innovations canalso advantageously be used in larger motors. The disclosed innovativecontrol arrangement can be used in combination with a wide variety ofswitching devices, including bipolar or MOS transistors (alone or incombination with shunt diodes), thyristors, relays, etc.

Similarly, the particular connections used for analog sensing are notparticularly critical. As will be obvious to those skilled in the art,the comparator inputs may be protected with series and/or shuntresistors and/or zener diodes, depending on the motor operating voltageand the expected exposure to transient voltages.

The foregoing description has focussed on the startup phase of motorcontrol. However, of course, there are a vast number of circuits andmethods for controlling DC motors, in response to changing loadconditions and operator demands, which can be used in combination withthe disclosed start-up procedures. These further control procedures canbe used in combination with the disclosed innovative startup procedures,and may even be implemented (at least partly) in shared hardwarecomponents.

Of course, the first and second time durations can be modified fordifferent motors, or at least for different classes of motors. Moreover,it is alternatively and less preferably possible to use more complexprocedures to adaptively modify these parameters, in service, to providethe best fit of these parameters to the particular motor/loadcombination being driven.

For another example, the disclosed innovations would be equallyapplicable to a DC motor which used a slip-ring (not commutated)arrangement to feed a rotor coil, instead of a permanent-magnet rotor.

The disclosed circuitry would be particularly well suited to a "smartpower" chip, which included the control logic shown together with theswitching transistors to drive a small motor.

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given.

What is claimed is:
 1. A method for starting-up a motor having multiplestator winding excitation phases separately excitable in a predeterminedsequence corresponding to a forward direction of rotation of a rotor,comprising the steps of:(a.) providing current to said windings, for afirst maximum time duration, to excite an initial phase thereof; whereinsaid first duration is sufficiently short that the rotor travels, duringsaid first duration, through no more than one-half of the angularseparation between two adjacent ones of the equilibrium positions,stable or unstable, of all of said phases; (b.) monitoring backelectromotive force (back EMF) on multiple ones of said windings, andgenerating digital signals accordingly; (c.) IF a zero-crossing of backEMF occurs on any one of said windings within at most a second timeduration, then using said digital signals to determine an optimal phasefor excitation, and exciting said optimal phase; and (d.) if NOzero-crossing of back EMF occurs on any one of said windings within saidsecond time duration, then repeating said step(a) with a differentinitial phase which is not adjacent to said first initial phase in saidpredetermined sequence of excitation phases; whereby said motor isstarted and operated without requiring sensor inputs to ascertain rotorposition, and with minimal backward rotation at the time of startup. 2.The method of claim 1, further comprising the preliminary step, prior tosaid step (a), of rapidly exciting multiple ones of said excitationphases.
 3. The method of claim 1, wherein said step of monitoring backEMF is performed by a plurality of differential amplifiers andhysteresis gates, each connected to a respective one of said windings.4. The method of claim 1, wherein said step of monitoring back EMF isperformed by a plurality of differential amplifiers and Schmitttriggers, each connected to a respective one of said windings.
 5. Themethod of claim 1, further comprising the additional step, after saidstep (d), of: continuing to supply successive phases of said windingswith current in said predetermined sequence, to maintain rotation ofsaid motor.
 6. The method of claim 1, wherein said step of using saiddigital signals to determine an optimal phase for excitation consistsessentially of using said digital signals as address inputs to a lookuptable.
 7. A method for operating a motor having multiple stator windingsand a rotor, comprising the steps of:(a.) providing current to saidwindings, for a first maximum time duration, to excite an initial phasethereof; wherein Said first duration is sufficiently short that therotor travels, during said first duration, through no more than one-halfOf the angular separation between two adjacent ones of the equilibriumpositions, stable or unstable, of all of said phases; (b.) monitoringback electromotive force (back EMF) on multiple ones of said windings,and generating digital signals accordingly; (c.) IF a zero-crossing ofback EMF occurs on any one of said windings within at most a second timeduration, and if the signs of said back EMF values after saidzero-crossing are consistent with the signs of said back EMF valuesbefore said zero-crossing, then using said digital signals to determinean optimal phase for excitation, and exciting said optimal phase; and(d.) if NO zero-crossing of back EMF occurs on any one of said windingswithin said second time duration, then repeating said step(a) with adifferent initial phase which is not adjacent to said first initialphase in said predetermined sequence of excitation phases; whereby saidmotor is started and operated without requiring sensor inputs toascertain rotor position, and with minimal backward rotation at the timeof startup.
 8. The method of claim 7, further comprising the preliminarystep, prior to said step (a), of rapidly exciting multiple ones of saidexcitation phases.
 9. The method of claim 7, wherein said step ofmonitoring back EMF is performed by a plurality of differentialamplifiers and hysteresis gates, each connected to a respective one ofsaid windings.
 10. The method of claim 7, wherein said step ofmonitoring back EMF is performed by a plurality of differentialamplifiers and Schmitt triggers, each connected to a respective one ofsaid windings.
 11. The method of claim 7, further comprising theadditional step, after said step (d), of: continuing to supplysuccessive phases of said windings with current in said predeterminedsequence, to maintain rotation of said motor.
 12. The method of claim 7,wherein said step of using said digital signals to determine an optimalphase for excitation consists essentially of using said digital signalsas address inputs to a lookup table.
 13. A method for starting-up abrushless and sensorless DC motor having multiple stator windings and arotor, without causing significant backward rotation, which comprisesthe steps of:(a) temporarily exciting the windings, in a first initialphase thereof, for a first interval of time; wherein said first intervalhas a maximum duration which is sufficiently short that the rotortravels, during said first interval, through no more than one-half ofthe angular separation between two adjacent ones of the equilibriumpositions, stable or unstable, of all of the phases of said windings;(b) after said excitation step (a), digitally reading the signs ofrespective back electromotive forces (back EMF)on multiple ones of thewindings; (c) detecting the occurrence of a first zero-crossing event inany one of said induced back electromotive forces within a preset secondinterval of time after said first excitation step, and accordingly: ifsuch a zero-crossing event occurs within said second interval, againdigitally reading the signs of said induced back electromotive forces,and utilizing said readings for decoding an optimal phase forexcitation; if such occurrence is not detected before the elapsing ofsaid second interval of time subsequent to the instant of interruptionof said first excitation step, repeating said steps (b) and (c) with asecond initial phase which is functionally shifted by at least two phasepositions in respect to said first initial phase; and (d) starting-upthe motor by sequentially exciting phases of said windings, initiatingfrom said decoded optimal phase.
 14. The method of claim 13, whereinsaid step of monitoring back EMF is performed by a plurality ofdifferential amplifiers and hysteresis gates, each connected to arespective one of said windings.
 15. The method of claim 13, whereinsaid step of monitoring back EMF is performed by a plurality ofdifferential amplifiers and Schmitt tiggers, each connected to arespective one of said windings.
 16. A method as defined in claim 13,further comprising the preliminary step, prior to said step (b), of:sequentially exciting all the phases of the motor, each for a fractionof said first interval of time for eliminating static component offriction without moving the rotor from its rest position, beforeperforming said first excitation step of said predefined phase.
 17. Amethod as defined in claim 13, wherein said back electromotive forcesinduced by the rotor and the motor's windings are digitally read byamplifying each a signal of induced back electromotive force by means ofa differential amplifier, feeding the amplified signal to an input of acomparator circuit having an output terminal capable of assuming a logicstate in function of the sign of the back electromotive force inducedpresent on the respective winding and by attributing a given weight tothe bit representing the output logic state of each comparator; thedigitally readable output configuration of the comparators correspondingto a certain angular position of the rotor while it is rotating in agiven sense of rotation and assuming a complementary logic value whenthe rotor travels through the same angular position while it is rotatingin an opposite sense of rotation.
 18. A method as defined in claim 17,further comprising the step of: utilizing said first reading fordecoding the row of a look-up table organized as a matrix of rows andcolumns and using said second reading for decoding the column of saidlook-up table, thus identifying the code of the phase to be excited nextfor accelerating the rotor in the desired sense of rotation with amaximum impulse of torque.
 19. A method as defined in claim 17, whereinsaid first interval of time is shorter than the time necessary for saidrotor to cause any two zero-crossings in the sign of any of the backelectromotive forces induced in the motor's windings.
 20. A circuit forstarting-up in a desired sense of rotation a multiphase brushless andsensorless DC motor having multiple stator windings and a rotor, whichcomprisesfirst means capable of producing a logic signal representativeof the sign of back electromotive forces induced in the motor's windingsby the rotation of the rotor; second means capable of generating asignal representative of the occurrence of an inversion of sign of anyone of said induced back electromotive forces; means for decodingthrough a look-up table of codes of excitable phases of the motorcapable of identifying an optimal phase to be excited for acceleratingthe rotor in a desired direction in function of said logic signalsproduced by said first means before and after said first zero-crossingevent as detected by said second means; at least a first timer startedby a start signal and capable of causing the excitation, for a presetfirst interval of time, of a predefined phase or of a phase functionallyshifted by two phase positions from said predefined phase; wherein saidfirst duration is sufficiently short that the rotor travels, during saidfirst duration, through no more than one-half of the angular separationbetween two adjacent ones of the equilibrium positions, stable orunstable, of all of said phases; a second timer started by the signalproduced by said second means and capable of enabling said first timerto excite said phase shifted by two phase positions from said predefinedphase in the case said second means fail to generate said signal beforethe elapsing of a second interval of time.
 21. A circuit as defined inclaim 20, wherein said first means are a plurality of signal processingcircuits composed of a differential amplifier and a comparator, eachdifferential amplifier and comparator circuit being able to produce onan output node of said comparator said logic signal representative ofthe sign of the back electromotive force induced in a respective windingof the motor.
 22. A circuit as defined in claim 21, wherein said secondmeans comprise a transition detecting circuit capable of detecting atransition of the output voltage of said comparators and of producing asignal representative of such a detected occurrence.
 23. An integratedcircuit, comprising a circuit in accordance with claim 20 integrated ona single chip together with power transistors.
 24. The method of claim1, further comprising a delay interposed between said steps (a) and (b).25. The method of claim 7, further comprising a delay interposed betweensaid steps (a) and (b).
 26. The method of claim 13, further comprising adelay interposed between said steps (a) and (b).